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Electrolyte and Dissociation

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Electrolyte and Dissociation Seminar 4 Conductivity and Conductometry. Electrolyte properties. Electricity flow through the electrolyte. Conductivity of electrolyte solutions (specific, molar) and its dependence on concentration. Determination of the limiting molar conductivity. Kohlrausch’s law. The use of electrolyte conductivity for physicochemical calculations (dissociation constant and degree, solubility product). Examples of conductometric titration (conductometry). Examples of graphs, tasks and their solutions. Tomasz Tuzimski Medical University of Lublin, Faculty of Pharmacy with Medical Analytics Division, Chair of Chemistry, Department of Physical Chemistry, 4A Chodźki Street, 20-093 Lublin, Poland E-mail: [email protected] Electrolyte and Dissociation An electrolyte is a substance that, when molten or dissolved in a solvent, breaks down into free ions (dissociates) as a result of which it can conduct electricity. Their ability to conduct electricity suggests the presence of electrically charged particles that are able to move through the solution. The generally accepted reason is that when an ionic compound dissolved in water, the ions separate from each other and enter the solution as more or less independent particles that are surrounded by molecules of the solvent. The change is called the dissociation of the ionic compounds. Dissociation of an ionic compound as it disolves in water. Ions separate from the solid and become surrounded by the molecules of water. The ions are said to be hydrated. Equations for dissociation reactions show the ions Acid dissociation constant Ka An acid ionization constant = An acid dissociation constant Base dissociation constant Kb Base ionization constant = Base dissociation constant Remember: you can't use the dissociation constant formula for strong electrolytes! Dissociation degree α Studies on electrolytes have shown that, despite the same molar concentration of different electrolytes in solution, they differ in their ability to conduct electricity. This means that in the solutions of different electrolytes, different numbers of molecules dissociate. The quantity that describes the quantitative dissociation of electrolytes in solution is the dissociation degree (α). Dissociation degree α Dissociation degree is the ratio of the number of dissociated molecules to the total number of molecules dissolved in a given solution (for a weak electrolyte solution). number of moles of dissociated molecules α= x 100% number of moles of molecules dissolved in a given solution Dissociation degree α The dissociation degree depends on: ♦ the type of electrolyte and type of solvent ♦ the concentration of the solution ♦ from temperature ♦ from the presence of other electrolytes in the solution. Types of electrolytes (0.1 M/L solutions) Strong Medium Weak STRONG ELECTROLYTE MEDIUM ELECTROLYTE WEAK ELECTROLYTE ≥ 0.3 0.05 ≤ ≤ 0.3 ≤ 0.05 ≥ 30% 5% ≤ ≤ 30% ≤ 5% Majority of salt H3PO4, HClO2, Mg(OH)2, Some inorganic acids (except for some mercury, H3AsO4 (H2CO3, HNO2, H2SO3, HClO, zinc and cadmium salts) HBrO, HIO, CH3COOH, H2S, HCN) Some inorganic acids Metal hydroxides (e.g. HCl, HNO3, H2SO4, HBr, (except group 1 and 2 HI, HClO4, HClO3) of the periodic table) AgOH and hydroxides of alkali Organic acids and bases and beryllium with the except oxalic and sulfonic exception of acids Be(OH)2 and Mg(OH)2 Examples of reactions Strong Medium Weak STRONG ELECTROLYTE MEDIUM ELECTROLYTE WEAK ELECTROLYTE 2+ - Mg(OH)2 Mg + 2OH NaCl → Na+ + Cl- H3PO4 CH COOH + - 3 HCl →H + Cl - + CH3COO + H 3- + PO4 + 3H NH4OH NaOH → Na+ + OH- + - NH4 + OH → dissociation of rather balance equilibrium an ionic compound of reaction is is complete shifted to the left Many ionic compounds have low solubilities in water. An example is salt AgBr. Although only a tiny amount of this compound dissolved in water, all of it that does dissolve is completely dissociated. However, because of the extremely low solubility, the number of ions in the solution is extremely small and the solution doesn’t conduct electricity well. Neverthless, it is still convenient to think of AgBr as a strong electrolyte because it serves to remind us that salts are completely dissociated in aqueous solutions. Solubility rules ➢ Due to the type of cation - sodium, potassium, ammonium salts are soluble. ➢ Due to the type of anion - all are soluble: • nitrates (V) except SbONO3, BiONO3; • nitrates (III) except AgNO2, • acetates except CH3COOAg, (CH3COO)2Hg2 Furthermore, the majority of: • chlorides are soluble in water except AgCl, Hg2Cl2, PbCl2, TlCl; • sulphates (VI) are soluble in water except BaSO4, SrSO4, PbSO4, Hg2SO4. In addition, most (IV) sulfates are insoluble in water except (NH4)2SO3 and alkali sulfates (1A periodic table of elements). Carbonates, phosphates (V) and sulphides of most metals are insoluble or sparingly soluble in water. Hydroxides are generally sparingly soluble in water, in addition to alkali hydroxides and fairly soluble Ba(OH)2 and partially soluble Ca(OH)2 and Sr(OH)2. Nonelectrolytes Aqueous solutions of most molecular compounds do not conduct electricity, and such solutes are called nonelectrolytes. Examples are sugar and ethylene glycol (the solute in antifreeze solutions). Both of these solutes consist of molecules that stay intact when they dissolve. They simply intermigle with water molecules when their solutions form. The solution contains no electrically charged particles, so it is unable to conduct electricity. Electrical conductivity of solutions of electrolytes vs nonelectrolytes The copper sulfate solution is a Neither sugar nor water is an strong conductor of electricity, electrolyte, and this sugar and CuSO4 is a strong electrolyte. solution is a nonconductor. Ohm's second law The resistance of the metal conductor is directly proportional to the length of the conductor and inversely proportional to its cross-section: where l is the length of the conductor in SI units of meters, S is the cross-sectional area in units of meters squared, and "ρ" (Greek "rho") is the resistivity in units of ohm·meters. If l = 1 m and S = 1 m2, then ρ (Greek "rho") is a characteristic value for a given conductor and is called its specific resistance expressed in units of ohm·meter. Specific conductivity „κ” (Greek „kappa") In many cases it is more convenient to use the inverse of the specific resistance, i.e. the specific conductivity „κ” (Greek „kappa") : 1 κ= ρ which is the conductivity of a cube with an edge of 1 m. Therefore, the dimension of specific conductivity is unit of ohm-1·meter-1. For practical reasons, the conductivity expressed as the conductivity of a cube with a 1 cm edge is more often used. Since the unit of conductivity is simens (S) defined as 1 S = 1 ohm-1, then: Electricity flow through the electrolyte Electrolytes conduct electricity due to the movement of ions in the electric field. The movement of ions in the solution may be limited, especially in strong electrolyte solutions with high concentrations, their interaction. For example, a negative ion is surrounded by, due to Coulomb electrostatic interactions, a cloud of positive ions that inhibit the movement of the central ion towards the anode, because the surrounding cloud tends to the cathode. This is called electrophoretic effect (Fig. b). Another phenomenon limiting the movement of ions is the relaxation effect, which means that the formation and disappearance of the ion cloud around the central ion requires some time. Behind the moving negative ion towards the anode, an ion cloud is formed from the ions of the opposite sign, while in the front the cloud has not yet been formed, or is small due to the movement of the ion. Such an asymmetrical system exists only when an electric field exists. If the electricity turns off, in a very short time, called relaxation time, the central ion will restore the spherical symmetry of its ion cloud (Fig. a). The dependence of the specific conductivity „κ” (Greek „kappa") on the concentration of different electrolytes (weak and a strong electrolyte solutions) Molar conductivity Λ (Greek "lambda") A more convenient measure of electrolyte conductivity, especially for the purpose of interpreting phenomena associated with the flow of ions in solution, is the so-called molar conductivity Λ (Greek "lambda") determined by the following formula: If the concentration is expressed in moles / dm3 (M/L), then: Linear and non-linear relationships Based on the measured molar conductivity values, or rather calculated from the specific conductivity and molar concentrations of solutions, it can be concluded that generally two types of curves are obtained: almost linear relationships for strong electrolytes, (e.g., HCl, H2SO4, NaCl) and non-linear relationships for weak electrolytes whose molar conductivity when diluted increases faster than for strong electrolytes. This rapid increase in molar conductivity associated with dilution of the solution is explained by the increasing dissociation rate of the weak electrolyte, which in turn is not the case with strong electrolytes. The dependence of the molar conductivity Λ (Greek "lambda") on the concentration for weak electrolyte (for acetic acid solution). The dependences of the molar conductivity Λ (Greek "lambda") on the concentration of strong electrolytes. Limiting molar conductivity Λ0 From the Kohlrausch’s formula, can be calculated so-called limiting molar conductivity Λ0 (Greek "lambda„ zero) value: If for an infinitely great dilution, that is, when the concentration is approximately equal to 0 (c = 0), i.e. one in which practically moving ions do not interact by Coulomb forces (and it is known that these interactions decrease with the square of the distance): Limiting molar conductivity Λ0 (strong electrolytes) Comparison of limiting molar conductivity Λ0 values of strong electrolytes Potassium Λ Sodium Λ Example Anion 0 0 Difference salts KA salts NaA 1 Cl− KCl 130 NaCl 108.9 21.1 - 2 NO3 KNO3 126.3 NaNO3 105.2 21.1 2- 3 SO4 K2SO4 133 Na2SO4 111.9 21.1 Kohlrausch's law independent ion migration Kohlrausch's law - the law according to which electrolyte conductivity, (molar or equivalent) is an additive quantity.
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